1. Field of the Invention
This invention generally relates to an electromagnetic clutch, such as for use in controlling the transmission of power from an automobile engine to a refrigerant compressor in an automobile air conditioning system. More particularly, this invention relates to the transmission of power from the automobile engine to a friction surface of the electromagnetic clutch.
2. Description of The Prior Art
An embodiment of Japanese Utility Model Application Publication No. 52-151258 is essentially illustrated in FIG. 1. Electromagnetic clutch 10' is intended to be coupled to a refrigerant compressor in an automobile air conditioning system. Compressor housing 11 is provided with cantilevered tubular extension 12 surrounding an extension of drive shaft 13 of the refrigerant compressor. Drive shaft 13 is rotatably supported in compressor housing 11 by bearings (not shown). The X-axis is the horizontal axis about which hub 24, annular armature plate 26, and clutch rotor 15 rotate.
Clutch rotor 15 is rotatably supported on cantilevered tubular extension 12 through bearing 16. Bearing 16 is mounted on the outer surface of cantilevered tubular extension 12. Clutch rotor 15 is made of a magnetic material, such as steel, and comprises outer annular cylindrical portion 151, inner annular cylindrical portion 152, and axial end plate portion 153. Axial end plate portion 153 connects outer cylindrical portion 151 with inner cylindrical portion 152 at each of their axial forward ends (to the right in FIG. 1). Axial end plate portion 153 includes first friction surface 153a. Annular U-shaped cavity 17 is defined by portions 151, 152 and 153. A plurality of V-shaped grooves 18 are provided on outer peripheral surface of outer annular cylindrical portion 151 for receiving belt 40. Belt 40 couples clutch rotor 15 to the power output of an automobile engine (not shown).
Axial end plate portion 153 includes one or more slits 19 which are disposed on a circle or on concentric circles centered about the X-axis. These slits 19 define a plurality of annular or arcuate magnetic pieces on axial end plate portion 153. The surface of the poles of the magnetic pieces are located on axial end plate portion 153.
Electromagnetic coil 20 is disposed in annular cavity 17 of clutch rotor 15 to supply a magnetic flux, shown by dashed line "M", for attracting annular armature plate 26 to axial end plate portion 153 of clutch rotor 15. Electromagnetic coil 20 is contained within annular magnetic housing 21 which has a U-shaped cross section. The annular magnetic housing 21 is fixed to supporting plate 22, which is secured to the axial forward end surface of compressor housing 11 by a plurality of rivets 221. A small air gap is maintained between annular magnetic housing 21 and clutch rotor 15.
Hub 24 is disposed on the terminal end of drive shaft 13. Hub 24 is secured to drive shaft 13 by nut 25. Hub 24 comprises tubular member 241 secured on the terminal end of drive shaft 13 and flange portion 242 extending radially from the axial end of tubular member 241. Flange portion 242 is integrally formed with tubular member 241. Alternatively, flange portion 242 may be formed separately from tubular member 241 and fixed on tubular member 241 by any known securing method, for example, welding.
Annular armature plate 26 is composed of a magnetic material and is concentric with hub 24. Annular armature plate 26 faces axial end plate portion 153 with predetermined axial air gap "1" in them. Annular armature plate 26 is connected elastically to flange portion 242 of hub 24 through a plurality of leaf springs 27. Annular armature plate 26 includes second friction surface 26a which faces first friction surface 153a which is located on axial end plate portion 153 of clutch rotor 15. Stopper plate 28 and one end of each leaf spring 27 are secured by rivets 29 to outer surface of flange portion 242 through spacing member 30. The other end of each leaf spring 27 is fixed to annular armature plate 26 by rivet 31 so that annular armature plate 26 is flexibly supported for axial movement upon deflection of leaf springs 27.
Thus, when electromagnetic coil 20 is energized, annular armature plate 26 is attracted to axial end plate portion 153 of clutch rotor 15. Consequently, first friction surface 153a and second friction surface 153a engage each other. Drive shaft 13 is then rotated together with clutch rotor 15 through leaf springs 27 and hub 24. The rotation is provided by the engine output.
When electromagnetic coil 20 is not energized, annular armature plate 26 is separated from clutch rotor 15 by the elasticity of leaf springs 27. Clutch rotor 15 is rotated by the engine output, but the refrigerant compressor is not driven.
To enhance the torque transmission from the clutch rotor to the annular armature plate, annular friction member 60', made of a nonmagnetic material, is fixedly disposed within annular groove 26b'. Annular groove 26b' is formed near the radially outermost edge of second friction surface 26a of annular armature plate 26. A first end surface (to the left in FIG. 1) of annular friction member 60' projects slightly with respect to second friction surface 26a of annular armature plate 26. However, the first end surface of annular friction member 60' can be considered to be substantially flush with second friction surface 26a of annular armature plate 26. The thickness of annular friction member 60' is required to be relatively large in order to easily form annular friction member 60'. Therefore, the depth of annular groove 26b' is required to be relatively large and the thickness of annular portion 26c' is decreased. Thus, the magnetic resistance at annular portion 26c' of annular armature plate 26 is increased. Accordingly, the number of magnetic lines of flux which radially penetrate through annular portion 26c' of annular armature plate 26 is decreased, thereby creating the situation where the electromagnetic attraction generated by a unit of electric power becomes smaller.
The above problem can be solved by increasing the thickness of the annular armature plate. However, a serious disadvantage created by this solution is an undesirable increase in the weight of the electromagnetic clutch.
Another disadvantage of electromagnetic clutches in the prior art is that they do not address the reduction of the amount of leakage flux, shown by dotted line "B", at radial outer region 153b of axial end plate portion 153.